Longest consecutive sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given[100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is[1, 2, 3, 4]. Return its length:4.

Your algorithm shold run in O (N) Complexity.

Answer

public class Solution {    public int longestConsecutive(int[] num) {        if(num==null||num.length==0)        {            return 0;        }        int result=1;        HashSet<Integer> set=new HashSet<Integer>();        Map<Integer,Integer>map=new HashMap<Integer,Integer>();        for(int element:num)        {            set.add(element);        }        for(int element:num)        {            if(map.containsKey(element))            {                continue;            }            int end=element;            for(end=element+1;set.contains(end)&&!map.containsKey(end);end++);            int len=end-element;            if(map.containsKey(end))            {                len+=map.get(end);            }            result=Math.max(len,result);            for(int i=element;i<end;i++,len--)            {                map.put(i,len);            }        }        return result;    }}

Longest consecutive sequence